<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <form action="">
        <div>
            用户名:
            <input type="text" autofocus required pattern="[0-9][A-Z]{3}">
            <!-- autofocus默认待输入 获取焦点 -->
            <!-- JS正则表达式
                 pattern="[0-9][A-Z]{3}" -->
        </div>
        <div>
            邮箱:
            <input type="email" name="email" required multiple>
            <!-- required必填项 不能为空 -->

            <!-- multiple可填写多个地址用逗号隔开 -->
        </div>
        <input type="submit">
    </form>
</body>
</html>